The present invention relates generally to adaptive (e.g. "smart") structures, and more specifically to a system and method for controlling the shape of a sheet of electroactive material, such as laminated piezoelectric films, by using an electron beam gun to deposit charge on multiple electrodes attached to the frontside of the sheet.
Electroactive materials, including piezoelectric, magneto restrictive, and electroristrictive, are used in "smart" structures because they provide direct coupling between electrical energy and mechanical energy in real-time. Piezoelectricity was discovered by Pierre and Jacques Curie in the 1880's. Piezoelectric properties occur naturally in some crystalline materials (e.g. Rochelle salt), and can be induced in others by applying a "poling" process (e.g. in lead zirconate titanate (PZT) and in polyvinylidene fluoride, PVDF). The orientation of the DC poling field determines the orientation of the mechanical and electrical axes. After the poling process is complete, a voltage lower than the poling voltage changes the dimension s of a ceramic element as long as the voltage is applied. A voltage with the same polarity as the poling voltage causes additional expansion along the poling axis and contraction perpendicular to the poling axis. A voltage with the opposite polarity has the opposite effect: contraction along the poling axis, and expansion perpendicular to the poling axis. In both cases, the ceramic element returns to its poled dimensions when the voltage is removed from the electrodes.
Application of a mechanical stress to a piezoelectric material generates an electric field; conversely, application of an electric field generates a mechanical strain (e.g. expansion and/or contraction). This induced strain can be used to change the position or shape of a structure, and to apply forces, in real-time. Active control of a "smart" structure's position, correction of improper shape, and suppression of undesirable vibrations can, therefore, be realized by applying an electrical potential to actuators made of electroactive materials. The actuators can be discrete or continuous; embedded or attached to the surface. For example, actuators made of piezoelectric materials are used in ink-jet printers, loudspeakers, sonar transducers, optical stages, adaptive mirrors, and spark generators. Piezoelectric sensors include accelerometers, microphones, and sonar systems.
As shown in FIGS. 1-3, conventional UNIMORPH or BIMORPH (registered trade names of Morgan Matroc, Inc., Electro Ceramics Division, Bedford, Ohio) piezoelectric structures will bend in response to an applied electric field and can, therefore, be actively deformed into desirable shapes. UNIMORPH structures (see FIG. 1) bend when a single expander plate of piezoelectric material expands or contracts relative to the attached substrate (due to the offset neutral axis). Parallel BIMORPH structures have two expander plates polarized in the same direction (see FIG. 2). Series BIMORPH structures have two plates polarized in opposite directions (see FIG. 3). BIMORPH elements develop relatively large deflections compared to UNIMORPH structures. A parallel-type BIMORPH develops twice the deflection, for the same applied voltage, than the series-type BIMORPH, because the electric field across any individual layer is twice as high in the parallel-type design, than in the series-type design. Consequently, parallel BIMORPHS are preferred for piezoelectric actuators, and series BIMORPHS for piezoelectric sensors. Electrodes are applied to the piezoelectric layers, and conventionally have attached electrical leads. For uniaxial actuation, multiple layers of piezoelectric materials can be stacked together and laminated, so that they are mechanically connected in series and electrically connected in parallel. The displacement of the whole stack assembly is equal to the sum of the individual displacements.
An important example of "smart" structures is adaptive mirrors. These can be used by astronomers to correct for turbulence of the atmosphere, and for wavefront control of laser beams in optical systems for military applications. Additionally, space-based deployable optics are needed to satisfy the need for enhanced capabilities for remote sensing. In particular, the need arises for low cost surveillance satellites that can be quickly launched and positioned in orbit to monitor rapidly evolving events almost anywhere on the globe. However, the desire for low cost and flexibility is in opposition with the requirement for large apertures needed to ensure sufficient ground resolution and sensitivity. Such systems usually imply large, expensive launch vehicles to accommodate the size of the primary mirror. Development of a deployable mirror or antenna is one approach being considered to satisfy these conflicting requirements. Folded up and carried on a small booster, the deployable mirror or antenna would open to its full diameter after reaching orbit. Unfortunately, the inherent flexibility of such a device makes it difficult to achieve optical quality surfaces, and this approach has therefore not yet been proven feasible. A need exists, therefore, for a system and method for actively controlling the shape of space-deployable mirrors to obtain optical quality surfaces.
Fueled by both NASA and DoD interests, much of the on-going research in deployable optics is focused on precision assembly of rigid mirror segments. While this approach shows promise for achieving near term sensing goals, it is unlikely that such a technique is capable of achieving long term areal density goals (on the order of 1 kg/m.sup.2) needed to meet cost targets. Low areal densities can be achieved by using inflatable optics. However, this approach allows only limited shape correction by adjusting internal pressure, and suffers from the added complication of diffraction from passing light rays through a gaseous medium.
Traditionally, the shape of adaptive mirrors have been controlled by using 10s -100's of individual piezoelectric stacks, or UNIMORPH and BIMORPH piezoelectric bending actuators, each having their own electrical lead wire and individual power supply. However, for highly flexible, large structures requiring precision shape control, the number of individual actuators needed makes individual lead wires and dedicated power supplies impractical. This limitation has led investigators to consider crafting the entire mirror structure from electroactive materials, such as large sheets of laminated piezoelectric films, which have only one electrode on the backside connected to a wire. In this concept, excitation voltage is applied to the frontside by direct deposition of electrons emitted by an electron beam gun. Operated in a vacuum, the scanned electron gun eliminates the need for using attached lead wires, and replaces the individual power supplies with a single energy input device. With this technique, the electric charge is controlled by steering the electron beam to a specific location on the mirror, and by controlling the dwell time and refresh rate. Specific electron beam characteristics, such as beam diameter, beam energy, and beam current, can be individually controlled to optimize the performance.
Since the current loop is only closed at the point of electron beam incidence on the piezoelectric material, distributed shape correction can be achieved by scanning the gun across the bare surface of the piezoelectric film and then dwelling on the desired location (e.g. addressing), and possibly rastering the beam in a defined region about the desired location. Only a single backside electrode is needed to control the electric charge, and to close the current loop. In addition, secondary electron yield characteristics of the piezoelectric material enable either the addition or removal of charge from the surface by altering the electric potential of the single backside electrode. Since the electron gun can only be operated efficiently in vacuum environments, it is well suited for active control of deployable optics in space. The spatial resolution can be as small as the spot size of the electron beam, which can be as small as tens of microns.
U.S. Pat. No. 5,159,498, "Active Mirror Assembly", to Hubbard, discloses the use of an electron beam gun for depositing charge on the rear surface of a piezoelectrically active pellicle made of a pair of oppositely-poled bare PVDF films, with a conductive front mirror surface. Hubbard teaches changing the electron energy in real-time to actively control the PVDF film's shape by taking advantage of the dependence of secondary electron emission yield on incident electron energy. However, Hubbard's control process is inherently unstable in many regions of operation. Additionally, Hubbard does not teach the use of multiple electrodes attached to the frontside of the PVDF film to collect and uniformly distribute the electron charges deposited by the electron beam gun, which problems associated with "blooming" (see below).
The use of an electron beam gun to apply distributed electric charges to bare PVDF films is discussed in "Control of Distributed Actuators Without Electrodes", by J. A. Main, in Proc. of 1997 ASME International Engineering Congress and Exposition, November 14-19, Dallas, Tex., AD-Vol. 54, Adaptive Structures and Material Systems. Main discusses the use of a cross-ply 0/90 composite geometry of PVDF films having preferentially biased piezoelectric properties. However, Main teaches that permanent surface (e.g. frontside) electrodes are not required because the need for electrodes is "eliminated completely" when using e-gun control.
Unfortunately, a critical problem with depositing electrons directly on to a bare piezoelectric film is the unwanted spreading of electric charges (e.g. electrons), which form an amorphous cloud of electrons in the vicinity of the electron beam incident spot or rastered area. This "blooming" effect continuously increases the cloud's diameter until the electron current is stopped. Uncontrolled blooming can degrade the degree of control required for precision adaptive optics.
Against this background, the present invention was developed, which relates to a method and system for actively controlling the shape of a sheet of electroactive material, comprising one or more electrodes attached to the frontside of the sheet; and including means for generating, aiming, and depositing a beam of charged particles (e.g. electrons) onto the electrode(s), whereby the sheet changes its shape in response to an electric field created across the sheet by an accumulation of electric charge within the electrode(s). Use of an electrode on the frontside ensures a uniform distribution of the charge with a single point of e-beam incidence, thereby greatly simplifying the beam scanning algorithm and raster control electronics, and also reducing the problems caused by "blooming". By placing a distribution of multiple electrodes over the front surface of the piezoelectric film (or other electroactive material), this arrangement enables improved control over the distribution of surface electric charges (e.g. electrons) by creating uniform (and possibly different) charge distributions within each individual electrode. Removal or deposition of net electric charge can be affected by controlling the secondary electron yield through manipulation of the backside electric potential.